Radar is the technique of using radio waves to detect the existence of an object and then to find the object's position in relation to a known point. Air traffic control uses radar to track planes both on the ground and in the air, and also to guide planes in for smooth landings. Police use radar to detect the speed of passing motorists. The military uses it to detect an enemy and to guide weapons. NASA uses radar to map the Earth and other planets, to track satellites and space debris, and to help with docking and maneuvering of spacecrafts.
Radar systems typically operate by emitting, for example, microwave energy from a transmitting antenna in the form of a focused beam with known divergence angles. Some of this emitted microwave energy is reflected off objects in the beam's path and collected by a receiving antenna. This emitted and reflected energy is minimally affected by environmental conditions, such as in rain, fog, clouds, or time of day.
Most radar equipment is pulse-modulated, i.e. the radiation from a transmitting antenna is in the form of very short bursts or pulses of radio frequency (RF) energy. Each pulse is followed by a relatively long resting period during which the transmitter is switched off and the receiver is operating. Pulsed radar is used for the observation of objects located great distances, such as an aircraft several hundreds of kilometers from a radar station.
Continuous wave (CW) is another type of radar. CW relies on the “Doppler shift” in frequency to detect moving objects and to measure their speed. This phenomenon, known as the Doppler effect, occurs with radio waves as well as with sound waves. As an object approaches a radar antenna, the frequency of the signal reflected by the object is higher than that of the transmitted signal. Conversely if an object is moving directly away from the antenna the frequency of the reflected signal is lower than that of the transmitted signal. For stationary objects there is no change in the frequency of the reflected signal.
An improvement to CW radar is frequency-modulated continuous wave (FMCW) radar. FMCW radar utilizes a frequency-modulated waveform that is continuously transmitted from and received by a radar station. In FMCW radar, the difference in frequency between an emitted wave from a transmitter and a reflected wave from the object is calculated at a receiver. The receiver then provides an indication of the range of the object. FMCW radar not only measures range or distance to the object, but also the object's speed. The FMCW radar is capable of both long ranges, in the tens of kilometers, as well as short ranges, in meters.
However, FMCW systems cannot range objects in a near-field. The near-field, when discussing radio wave propagation with respect to radar, is an area in the immediate vicinity of a radar system's antenna. More specifically, it is a non-distinguishable or non-sensing area close to the radar system in which objects distance cannot be determined. In contrast, the far-field is an area that is far enough from the antenna to be approximated as a plane wave. Exactly how far away the transition from near-field to far-field occurs is a function of the bandwidth of a frequency sweep, the geometry and size of the antenna, and the electromagnetic properties of the target.
The behavior of wave propagation in the near-field is different than it is in the far-field. When objects are in the near-field, an FMCW radar system cannot distinguish between objects and noise. Hence, a typical radar system filters the data and removes the near-field effects in this non-distinguishable or non-sensing range. Accordingly, FMCW radars are not useful for ranging or detecting objects in the near-field.